1. Field of the Invention
The present invention relates to a wafer dividing apparatus for dividing a wafer along a plurality of crossing streets formed on the wafer in the condition where the strength of the wafer is reduced along the streets, and relates also to a laser processing apparatus including this wafer dividing apparatus.
2. Description of the Related Art
In a semiconductor device fabrication process, a plurality of crossing division lines called streets are formed on the front side of a substantially disk-shaped semiconductor wafer to thereby partition a plurality of regions where devices such as ICs and LSIs are respectively formed. The semiconductor wafer is cut along the streets to thereby divide the regions where the devices are formed from each other, thus obtaining the individual devices. Further, an optical device wafer is formed by laminating gallium nitride compound semiconductors or the like on a sapphire substrate or a silicon carbide substrate. The optical device wafer is also cut along the streets to obtain individual optical devices divided from each other, such as light emitting diodes and laser diodes, which are widely used in electric equipment.
Cutting of such a wafer including a semiconductor wafer and an optical device wafer along the streets is usually performed by using a cutting apparatus called a dicing saw. This cutting apparatus includes a chuck table for holding a workpiece such as a semiconductor wafer and an optical device wafer, cutting means for cutting the workpiece held on the chuck table, and feeding means for relatively moving the chuck table and the cutting means. The cutting means includes a rotating spindle, a cutting blade mounted on the rotating spindle, and a driving mechanism for rotationally driving the rotating spindle. The cutting blade is composed of a disk-shaped base and an annular cutting edge mounted on a side surface of the base along the outer circumference thereof. The cutting edge is formed by fixing diamond abrasive grains having a grain size of about 3 μm to the base by electroforming so that the thickness of the cutting edge becomes about 20 μm, for example.
However, the sapphire substrate and the silicon carbide substrate mentioned above have high Mohs hardness, so that cutting by the cutting blade is not always easy. Further, since the cutting blade has a thickness of about 20 μm, each street partitioning the devices must have a width of about 50 μm. As a result, the ratio in area of the streets to the wafer is increased, causing a reduction in productivity.
As a method of dividing a wafer along the streets, a laser processing method using a pulsed laser beam having a transmission wavelength to the wafer has been proposed. In this laser processing method, the pulsed laser beam is applied to the wafer along the streets in the condition where a focal point of the pulsed laser beam is set inside the wafer, thereby continuously forming a modified layer inside the wafer along each street as a break start point. Thereafter, an external force is applied to the wafer along each street where the modified layer is formed as the break start point to reduce the strength, thereby breaking the wafer along each street (see Japanese Patent No. 3408805, for example).
As a method of applying an external force to the wafer along each street where the modified layer is formed, thereby dividing the wafer into the individual devices, Japanese Patent Laid-open No. 2005-129607 discloses a method of expanding the dicing tape attached to the wafer to apply a tensile force to the wafer, thereby dividing the wafer into the individual devices.
However, this method of expanding the dicing tape attached to the wafer to apply a tensile force to the wafer has the following problem. When the dicing tape attached to the wafer is expanded, the tensile force is radially applied to the wafer, and this tensile force is accordingly applied to the crossing streets in random directions. As a result, the wafer may be divided irregularly and an undivided area may be left.
To solve this problem, Japanese Patent Laid-open No. 2006-40988 discloses a wafer dividing apparatus which can accurately and reliably divide a wafer along each street where the strength of the wafer is reduced. This wafer dividing apparatus includes a first suction holding member and a second suction holding member for holding the wafer through the dicing tape under suction in the condition where each street is interposed between the first and second suction holding members. In this condition, the first and second suction holding members are moved in opposite directions to thereby apply a tensile force to the wafer in a direction perpendicular to each street. As a result, the wafer can be divided along each street accurately and reliably.